You can plug the UPC codes from your supplier into Amazon (one at a time), and it should pull up the product. From there, you can then find the ASIN on the listing page under "Product Details" as shown below.
If multiple products are coming up for a single UPC, you should get in touch with your supplier and see which variation they are selling. You can ask for the exact name of the product, a picture of it, or if they have a link to their product listing on Amazon.
Additionally, if your supplier sends inventory in a spreadsheet format with UPC codes and prices then you can run the sheet through an analyzer tool and that will find all the ASIN’s for you.
My supplier only sent me UPC codes.
How do I turn a UPC code into an ASIN?